To form a thin paper web from a slurry of water and fiber, the wet web is dewatered, and then the dewatered web is at least partially dried. In the manufacture of tissue and similar paper products, creping is commonly used on such dewatered webs to impart desirable properties, such as softness and bulk. Creping is typically accomplished by conveying or carrying the web on a fabric to a heated rotary drum, sometimes referred to as a Yankee dryer. The web commonly is transferred to an adhesive dryer surface of the dryer and carried around a major circumferential portion of the dryer before the web reaches a zone of web de-contact from the drum. The de-contact zone is equipped with a creping blade against which the web abuts so as to be pushed backwardly or compacted upon itself in a machine direction of the web and attain the well-known tissue crepe paper structure, at which point the resulting creped web is removed from the dryer and collected, usually in rolled up form.
Before the web is transferred to the Yankee dryer, typically an adhesive composition, sometimes referred to as a “coating package” in the industry, is applied directly to the dryer surface of the dryer to form the adhesive dryer surface. The creping action typically requires some adhesion of the web to the outer surface of the dryer to effect a consistent and uniform creping action. Creping adhesives alone or in combination with release agents or other adjuvants have been applied either to the web or to the surface of the dryer in efforts to provide some balance of adhesion and release between the web and the dryer surface for purposes of drying and creping.
Various properties of the creping adhesive can be factors in the creping performance obtained. The level of adhesion of the creping adhesive to the drum dryer surface can be another factor which affects creping performance and results. Inadequate adhesion can result in poor creping, sheet floating, poor sheet handling, or other problems, whereas excessive adhesion may result in crepe blade picking, web plugging behind the crepe blade, web breaks due to excessive tension, or other problems.
Various types of creping adhesives have been used to adhere fibrous webs to rotary dryers such as Yankee dryers. Natural (unmodified) starch has been used in the past, which has been replaced by other chemistries over the years for many reasons. Natural starch cannot reach a high solid content with acceptable viscosity after cooking. Natural starch needs on-site cooking equipment and the equipment is expensive and needs significant maintenance work, such as runability and bacteria issues. Cooked natural starch contains tiny gel balls that make it hard to spray the material onto a Yankee surface and difficult to form a uniform film. Further, natural starch has limited Yankee protection ability and bonding strength. In place of natural starch, creping adhesives have included, for example, polyvinyl acetate-ethylene copolymer emulsions and aqueous polyvinyl alcohol solutions. It has been found that conventionally used polyvinyl acetate-ethylene copolymer compositions, which may contain small percentages of polyvinyl alcohol such as less than about 5% of the total solids by weight, may be generally adequate for the purpose but can cause a number of undesirable effects, such as blocking problems and others as mentioned in U.S. Pat. No. 6,991,707 B2, which is incorporated herein by reference in its entirety. Polyvinyl alcohol compositions (which may contain some polyvinyl acetate) can pose similar problems when used as creping adhesives, and can tend to coat the dryer with a hard and uneven film that builds up as drying and creping proceed, resulting in uneven creping or other problems.
Other creping adhesives have included wet strength resins, such as polyamide epichlorohydrin (PAE), poly(amidoamine) epichlorohydrin (PAAE), and polyethyleneimine (PEI). PAE and PAAE resins are described, for example, in U.S. Pat. Nos. 2,926,116; 7,404,875 B2; 7,943,705 B2; and 7,718,035 B2. PAAE based, wet strength resin or “WSR,” are not green materials. The product and by-products of the PAAE resins usually are not food grade, biodegradable, or renewable. For wet strength resin based Yankee coating chemistries, the performance of PAAE resins alone may not be good enough. For PEI based resins, the product cost is highly impacted by raw material costs.
There also is a need for coating formulations that are versatile enough to be used on conventional Yankee rolls, as well as with TAD fabrics and other modes of operation used for drying and creping wet tissue webs. In a typical through-air drying (TAD) operation, for instance, a web formed from a slurry of water and fiber is dewatered without significantly mechanically pressing the wet web, such as by vacuum drying. This is followed by a drying action using a hot air blast. The resulting webs can be transferred to a surface of a Yankee dryer for creping using a knuckled fabric so that the web adheres to the dryer in closely spaced zones, with bulking of the web between the zones. Alternatively, many desirable properties of creped tissue can be imparted without Yankee creping. For instance, a wet tissue web can be foreshortened while still moist in a process referred to as rush transfer, which can be used as part of a TAD operation without use of a Yankee drum. As shown for example in U.S. Pat. No. 5,888,347, a wet tissue web can be transferred from a forming section wire to a second slower-moving transfer fabric under carefully controlled conditions to foreshorten a moist paper web before it is transferred to a TAD fabric and through-dried to final dryness. The transferred moist tissue web can be placed on a TAD fabric that has a three-dimensional structure, after which hot air can be passed through the web to dry the web in a morphology corresponding to that of the TAD fabric, producing a web that can have improved softness without use of Yankee drying/creping.
Improved coating formulations are needed for tissue drying and creping processes (or other foreshortening processes), which can be sourced from high solid content, high temperature resistant and more environmentally-friendly adhesives that can provide an improved tack profile with respect to moisture content or other properties of the tissue web during pre-creping drying of the tissue and reduce coating costs with improved performance or without loss of performance.